The present invention relates to directly-heated oxide cathodes, which are used for fluorescent display tubes, light source units, embodying the principle of a fluorescent display tube, for large screen display, light sources for printers, and self-luminous devices for back-lighting. Particularly, the present invention relates to directly heated oxide cathodes, each which has good emission characteristics and low power consumption, and to fluorescent display tubes employing the same.
FIG. 22 is a perspective view illustrating a fluorescent display tube 1. FIG. 23 is a cross-sectional view illustrating the fluorescent display tube shown in FIG. 22. The fluorescent display tube 1 has a triode tube structure. A vacuum vessel is formed of an insulating glass substrate 2, a front glass 30, and a spacer glass 31. The vacuum vessel contains an anode electrode 24, a grid electrode 4 and filament cathodes 5 each acting as a directly heated oxide cathode. A through hole 22 is formed in the upper surface of the wiring layer 21 laminated over the upper surface of the insulating substrate. A conductive material 23 is buried in the through hole 22. The anode electrode 24 is laminated on the upper surface of the insulating layer via the conductive material. The filament cathodes 5, each of which emits thermal electrons, are arranged and supported by filament anchors. Electrical signals are externally supplied to the anode electrodes, the grid electrodes and the filament cathodes via the lead pins 6.
In order to fabricate the filament cathode 5, mixed carbonate powders 52 of alkaline earth metals (e.g. barium (Ba), strontium (Sr) and calcium (Ca)) are coated in a thickness of several xcexcm and on the surface of a very fine core metal wire 51 (e.g. tungsten or rhenium tungsten) of a diameter of 5 to 41 xcexcm. The filament cathodes 5 are disposed in the vessel of the fluorescent display tube. Thereafter, while the vessel is being evacuated in vacuum, the filaments are electrically heated to convert them into oxides. Thus, each directly heated oxide cathode is formed as an electron emissive source.
In the fluorescent display tube 1, the electrons emitted from the filament cathodes 5, which are accelerated by means of the grid electrode 4, impinge against the fluorescent substance layer 25 formed on the anode electrode 24. The accelerated electrons excite the fluorescent substance layer 25, light thus emitting from it.
In order to emit light from the fluorescent substance layer effectively, it is required that the filament cathode 5 has an improved emission capability and a reduced power consumption.
Generally speaking, when an oxide cathode is heated, the thermal electron flow (saturated current) per time emitted from the surface thereof is expressed by the following Richardson-Dushmann equation.                               I          s                =                  S          ⁢                      xe2x80x83                    ⁢          A          ⁢                      xe2x80x83                    ⁢                      T            n                    ⁢                      exp            ⁡                          (                              -                                                      e                    ⁢                                          xe2x80x83                                        ⁢                    φ                                                        k                    ⁢                                          xe2x80x83                                        ⁢                    T                                                              )                                                          (        1        )            
where
Is is a saturated current (a maximum current (A) derived from a material at a temperature);
S is a thermal electron emission area (cm2) of a cathode;
A is a thermal electron emission constant (A/cm2Kn) (either T2 in equation and K2 in units or Tn in equation and Kn in units so that the units balance in the equation);
T is the temperature of a cathode (K);
e is an electron charge;
xcfx86 is the work function (eV); and
k is the Boltzmann""s constant.
As apparent from the equation, in order to increase the saturated current density Is, there are three requirements: (1) higher cathode temperature, (2) larger thermal emission area and (3) smaller work function.
The work function xcfx86 is a value inherent value determined by the electron emissive material and by the fabrication method. When it is assumed that the work function xcfx86 of a ternary oxide of (Ba, Sr, Ca)O is about 0.9 eV and is constant for respective fluorescent display tubes, it is understood that increasing the thermal electron emission area S of the cathode and increasing the cathode temperature T can result in improving the cathode""s emission capability.
The thermal emission area S can be widened by increasing the oxide coating amount, that is, the coating thickness. However, because an increase of the oxide coating amount causes an increase of the radiant heat from the surface of the oxide cathode, the temperature affecting the emission capability may be decreased.
Some prior art publications disclose techniques in consideration of the above-mentioned problems. According to Japanese Patent Laid-open Publication No. 9-148066, the electron emissive area corresponds to the surface area of a filament cathode, on which a ternary oxide is coated. The ternary oxide, formed of barium, strontium and calcium, has a thickness of 6.5 to 7.5 xcexcm, so that a good emission capability can be obtained with a fixed power consumption and in a good balanced state.
Japanese Patent Laid-open Publication No. 60-63484 discloses a method of producing a pure oxide cathode, of which the grain size (diameter) is reduced, having good emission characteristics. This method includes the steps of making grains (or particles) of carbonate of alkaline earth metal while stirring and reacting an ammonium carbonate aqueous solution in a nitrate aqueous solution of an alkaline earth metal (such as barium (Ba), strontium (Sr) and calcium (Ca)) at high-velocity revolution; making an electro-deposit solution while mixing and dispersing the alkaline metal carbonate, a bonding agent, and an organic solvent; disposing, within a vessel, filament cathodes each formed of a tungsten (W) core wire having the surface on which the carbonates are deposited using the electro-deposit solution; and thermally decomposing the ternary carbonate, made of an alkaline earth metal, while evacuating the vessel in vacuum.
Because of the recent demand for energy saving, it has been required to reduce the consumption power of a fluorescent display tube. The problem of the present invention is to reduce the power consumed by the filament cathode, corresponding to 50% to 70% of the power consumption of a fluorescent display tube, and to reduce the power consumption of a fluorescent display tube using filament cathodes.
The inventors in the present application studied devotedly to reduce the power consumption of the fluorescent display tube. As a result, the inventors found that uniformly coating fine ternary carbonate particles on the surface of a core metal wire allows the power consumption of the filament cathode to be reduced.
Moreover, the inventors found that the power consumption consumed by the filament cathodes, corresponding to 50% to 70% of the total power consumption of a fluorescent display tube, can be reduced by using the filament cathode of the present invention.
The present invention is made to solve the above-mentioned problems.
An object of the invention is to provide a directly heated oxide cathode which has good emission characteristics and reduced power consumption.
Another object of the present invention is to provide a fluorescent display tube, which employs the above-mentioned cathode.
An aspect of the present invention relates to a directly heated oxide cathode in which an electron emissive material having a thickness of 0.5 xcexcm to 4.0 xcexcm is formed on a surface of a core metal wire.
Another aspect of the present invention relates to a directly-heated oxide cathode in which a solid solution of an electron emissive material formed of oxide crystal grains each having an average grain size of 0.1 xcexcm to 2.0 xcexcm is coated on a surface of a core metal wire.
A further aspect of the present invention relates to a directly-heated oxide cathode in which an electron emissive material is coated on a surface of a core metal wire, the electron emissive material being a solid solution formed of grains each having an average grain size of at least 0.1 xcexcm to 2.0 xcexcm, the electron emissive material having an average thickness of 0.5 xcexcm to 4.0 xcexcm.
A still further aspect of the present invention relates to a directly-heated oxide cathode in which an electron emissive material is coated on a surface of a core metal wire, the electron emissive material being formed of oxide crystal grains formed of carbonate particles each having an average grain size of at least 0.1 xcexcm to 2.0 xcexcm.
In the directly heated oxide cathode, the core metal wire comprises a tungsten core wire or a rhenium tungsten core wire.
In the directly heated oxide cathode, the electron emissive material comprises a ternary oxide formed of at least barium, strontium and calcium.
In the directly heated oxide cathode, the electron emissive material comprises a ternary oxide in which the weight ratio of barium (Ba), strontium (Sr) and calcium (Ca) is at least Ca:Sr:Ba=(5 to 25):(25 to 60):(30 to 60).
Still another aspect of the present invention relates to a fluorescent display tube including as a thermionic source the directly heated oxide cathode according to the present invention.